Order, acidity

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

The pA"a values of 49 derivatives of pyrazol-3-one were measured by poten-tiometric titration and their H NMR spectra were recorded in DMSO- fe- The experimental acidity order correlates for structurally similar compounds as do substituent constants and HMO electron densities (76JPR555). 

No. Overall order Order in alcohol Order in acid Order in catalyst... 

The gas-phase acidities of hydroxamic acid and its N- and O-methyl derivatives have been measured using FT ion cyclotron resonance (Table 15)160. The acidity order is the same as that found in DMSO although, in this solvent, the O-methylation of acetohydroxamic acid decreases the acidity by 1 pK unit and the IV-methylation by 3.6 pK units, respectively. 

The effect of the following groups In Increasing acidity order is... 

When different types of reactive sites are compared, however, there can be dramatic inversions of acidity order. Fluoride is a relatively weak base in solution, with pAjfHF) = 3.2 in aqueous solution. In contrast, in the gas phase, fluoride is strong enough to exothermically deprotonate acetone, which has a p of about 20... 

According to the global charge on each hydroxyl type, the acidity order for the... 

Notably, pKa values pertaining to solution equilibria are not usually distorted by special solvation effects. The same acidity order for 4-substituted pyridinium ions is found in the gas phase. In fact, a linear free energy (aqueous solution)-enthalpy (gas phase) relationship exists substituent effects are 3.5 times larger in the gas phase. In other words, the aqueous solvent, relative to the gas phase, attenuates the effect of substituents by its higher dielectric constant and by hydrogen bonding.52... 

The following groups of compounds illustrate the profound effect that resonance delocalization has on the stability of anions and hence the acidity of the conjugate acids. To compare the acidities of these acids, the conjugate bases can be ranked according to their resonance stabilization and that ranking of anion stabilization is predictive of the acidity orders. 

It should be noted at this point that there is a consistency in the pKa values measured in pure DMSO by the various techniques. However, there is some uncertainty at present relating to pKa values obtained by acidity function procedures in alcoholic and aqueous DMSO media. Thus one has the unexpected situation that theoretical analysis of the medium effect on p/ a is hampered because the values for a number of weak acids referring to the standard state in water are in doubt. Under these circumstances less than critical application of (6) to many weak acids, including some carboxylic acids, phenols as well as carbon acids, is inadvisable. Of course, (6) is strictly applicable to those cases in which the p/sfa values can be measured in the pure hydroxylic solvents and also for those weak acids which obey the criteria outlined by Cox and Stewart (1976). Despite this difficulty there is now a large body of reliable pKa data in both DMSO and water. Thus in principle it should be possible to account for variations, or reversals, in acidity order in terms of the thermodynamic transfer functions in (6). 

The Lewis acid order is SnCL > SnBr4 > Snl4. 

In most cases, the gas-phase acidity orders differ dramatically from those observed in solution since the Gibbs energies of solvation ca. 200-600 kJ/mol cf. Table 2-8 in Section 2.3) are much larger than the intrinsic acidity differences for most pairs of compounds. Thus, the relative acidities in solution are often dictated by the differential Gibbs energies of solvation rather than by the intrinsic properties of the solute molecules. 

A study of some C—H acids in dimethyl sulfoxide solution led to an acidity order that almost paralleled that found in the gas phase, whereas in protic solvents the order was different [116, 124, 125]. This result highlights the importance of specific solute/solvent interactions such as hydrogen bonding in comparing acid/base equilibria measured in the gas phase and in solution. 

Other well-studied cases of acidity order changes induced by differential solvation are substituted phenols [131] and halo-substituted carboxylic acids [34a, 122, 132, 239]. For a comparison of the acid/base behaviour of oxygen-versus sulfur acids and bases e.g. R—OH/R—SH) in the gas phase and in solution, see reference [214]. 

The gas-phase acidity order of haloacetic acids X—CH2—CO2H is H < F < Cl < Br < I, i.e. the order is reversed compared to that in aqueous solution [34a, 122, 132, 239]. Thus, the well-known aqueous acidity order is not caused by the increasing inductive substituent effect (I < Br < Cl < F), as is generally assumed, but rather by solvation effects. 

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